Support for query() in transactions

[czee]

It is possible to mix write() and query() calls in transactions. Only tested with Rigol instruments (DS1054, DG811, DP711).

As seen in the updated documentation:

Note that all query() calls need to return a string for the transaction. This is important when using a Facet since it is possible to make a Facet return an object.

Some instruments (Rigol DP711) do not support multiple commands for which transactions will not work as intended.

[czee]

Perhaps this could be refactored in some way to avoid duplication with the code below in query()

[natezb]

I don't think it makes sense to change the behavior of query() within transactions because normally you'd expect a query to return a result (not None). This is especially true when a transaction-unaware function or method is called within a transaction, which makes it hard to reason about composing functions. For example

class MyDevice(VisaMixin):
    def maybe_do_something(self):
        if self.query(':state?') == 1:
            do_something()

    def do_more_stuff(self):
        with self.transaction():
            self.maybe_do_something()  # This call behaves unexpectedly

If you find it useful to chain queries together, then maybe we should add an explicit way to do it? I'd be curious to see what you think.

[czee]

I'm not sure how you would ensure that query() returns something within a transaction since you have to string the whole message together first and then exit the transaction to flush the queue. This may require some decorator magic. Currently I use the functionality like this:

    with self.oscilloscope.transaction():
        self.oscilloscope.ppulses_min # query
        self.oscilloscope.ppulses_max # query 
        self.oscilloscope.scale1 = self.args.vertical_scale # write
        self.oscilloscope.timebase_main_scale = self.args.timebase_main_scale # write
    self.ppulses_min, self.ppulses_max = self.oscilloscope.response

As far as I can tell the existing functionality for query() calls in transactions is only to flush the message queue. In this change transaction query() calls will be returning data in the order in which they are called.

[czee]

I think you would need something to track the variable after it's already inside the transaction. Not sure if that's possible:

with self.oscilloscope.transaction():
    myquery_response = self.oscilloscope.ppulses_min # query